"You are creating a seam of disinformation which pushes the focus away from IRA involvement to British military involvement in this terrible atrocity."

Mr Byrne, a construction company owner who is in a relationship with a woman who used to be married to a member of the security services, denied this.

He said: "I am merely relating what I have been told. There is no desire to create disinformation."

Mr Byrne added that he still lies in bed reliving the memory of the atrocity and can remember seeing the bullet holes after gunmen opened fire on a remote road.

He told the inquest: "The smell of the scene was indescribable. It was the smell of death. The blood was running down the road."

Colleagues of Captain Nairac used the recent 40th anniversary of his death to distance him from any involvement in the Kingsmill massacre and other atrocities committed during Ulster's bloody Troubles.

While the intelligence officer's exact role in the late 1970s is still a mystery, one of the theories behind his kidnap from a Republican-supporting pub inside the Northern Irish border is that

his cover was blown after infiltrating the IRA.

It is accepted that he was tortured and executed in the Republic of Ireland the following morning with his body believed to be buried nearby in dense Ravensdale Forest.